Offshore drilling rigs, such as fixed platforms, jack-up or semi-submersible platforms, and drill ships, used in hydrocarbon production, normally use a riser to connect the rig with a wellhead at the seabed. In use, the riser keeps water from the drilling string and conveys circulated drilling mud. Typically, the riser has sections of metal pipe that are positioned vertically between the rig and wellhead. These pipe sections include peripheral auxiliary lines and pipes for communicating hydraulic lines between the rig and a blowout preventer at the wellhead.
The significant weight of steel risers is one drawback that limits their use in deep-sea operation. As is know, each of the steel pipe sections of the riser must have an adequate wall thickness to handle working pressures and to withstand the tensile load of other sections. These requirements add weight to the riser string. In turn, the weight of the riser string can be substantially limited to payload capacity of the floating rig that can only carry a limited number of sections without exceeding its maximum load limit.
As an alternative to the use of steel, an aluminum riser known in the prior art uses sections of aluminum pipe serially coupled together by flange connectors at the ends of the pipe. An example of such an aluminum riser is disclosed in U.S. Pat. Nos. 6,415,867 and 6,615,922. These flange connectors have openings for bolts and threaded inserts to connect the flange connectors together and have openings for carrying auxiliary pipes longitudinally along the pipe's periphery. To make a reliable connection, operators must tighten each bolt with a specified torque. Some riser designs may have anywhere from 6 to up to 18 bolts per connection. Consequently, assembling the sections of pipe can takes significantly operators considerable time to complete and verify.
In yet another drawback, the prior art riser assembly is made from the aluminum alloy 1980 T1 OCT 192048-90 (i.e., an aluminum alloy known as Russian Designation AL 1980 T1). (The “T1” designation is an equivalent to “WP” as described in R 0067—Alloy Temper Designation System for Aluminum (ANSI H35.1—2000). The letter “W” signifies “Solution Heat Treated”). For such thermo-strengthened alloys, the weld must be heat treated after welding. This makes it more difficult to fabricate the joints because the heat treatment procedure demands additional production time, personnel and equipment.